Nutrient cycling in a cropping system with potato, spring wheat, sugar beet, oats and nitrogen catch crops. I. Input and offtake of nitrogen, phosphorus and potassium

Nutrient balances, defined as the difference between input with manures, fe rtilizers and atmospheric deposition and offtake of nutrients with harveste d products in arable cropping systems, need to be positive to compensate fo r unavoidable losses to the environment, but should be kept at the lowest p ossible level to minimize emissions or unnecessary accumulation of nutrient s in the soil. Data from five consecutive years are reported from a long-te rm nutrient monitoring experiment with three replicates, managed comparably to conventional farming practice. There were four nutrient treatments (T1- T4). Treatment T1 received chemical fertilizer only. T2 received processed organic manure, supplying 50 per cent of the crop N-requirement, supplement ed by chemical fertilizers. In treatments T1 and T2 the soil was bare durin g winter. In T3 and T4 the crops were fertilized as in T1 and T2, respectiv ely, but nitrogen catch crops were grown in autumn and winter. Averaged ove r five years, the N-balances were 46 kg N ha(-1) y(-1) in T1 and T2 and 25 kg ha(-1) y(-1) in T3 and T4 (atmospheric deposition of 44 kg N ha(-)1 y(-1 ) included). Averaged over all treatments and years, the P-balance was 7 kg ha(-1) y(-1) and the K-balance -33 kg ha(-1) y(-1). The initially high soi l fertility indices for both P and K declined over the experimental period. Catch crops and organic manure did not affect crop yields or nutrient bala nces, except that their combination in T4 resulted in 1.5 ton ha(-1) extra dry matter yield of sugar beet roots. Between spring and harvest, potato an d sugar beet showed positive N balances and the cereals negative N-balances . Sugar beet was the only crop with a positive K-balance. NPK concentration s in plant products were not systematically affected by treatments but vari ed considerably between seasons. At harvest, on average 63, 71, 75 and 112 kg N ha(-1) (0-90 cm) were found after sugar beet, spring wheat, oats and p otato, respectively. In November catch crops accumulated on average 39 kg N ha(-1) after cereals and 33 and 5 kg ha(-1) after potato and sugar beet, r espectively. In March catch crops after the cereals contained 4 kg N ha(-1) less than in autumn, but after potato and sugar beet N-accumulation in spr ing had increased to 49 and 29 ha N ha(-1), respectively. In spring soil mi neral N (0-90 cm) varied across years from 31 to 63 kg ha(-1). The results indicate that compliance with a maximum excess of input over offtake, as im posed by future legislation, is feasible for N for cropping systems compara ble to the system examined, but that the standard for P will probably turn out to be a tight one.